AutoDesk web service A360 seems to be more picky than the AutoCAD desktop applications, may be it helps to use the
latest DXF version supported by ezdxf, which is DXF R2018 (AC1032) in the year of writing this lines (2018).

If you are adding XREFS and IMAGES with relative paths to existing drawings and they do not show up in AutoCAD
immediately, change the HEADER variable $PROJECTNAME='' to (not really) solve this problem.
The ezdxf templates for DXF R2004 and later have $PROJECTNAME='' as default value.